Method for producing seed crystals



Jan. 5, 1965 E. M. VAN DORNICK METHOD FOR PRODUCING SEED CRYSTALS Filed April 8, 1963 23 E wneo M. VM/DQN/CK a l/45 f United States Patent "ice.

3,164,328 METHOD FOR PRODUGNG SEED CRYSTALS Edward M. Van Dornich, 3716 E. Corta Calla, Pasadena, Calif. Filed Apr. 8, 1963, Ser. No. 271,138 4 Claims. ('81. 24121) This invention has to do generally with the continuous production of crystallized materials, and is directed particularly to improved processes for assuring formation of product or crop crystals having desirable uniformity in size, with reduced fines concentration in the crystallizer.

In most industrial crystallizations, the crystal growth and seed nucleation (initial particle precipitation from solution) occur simultaneously and continuously. Consequently, during the period of growth of earlier formed seed crystals to the desired product size, the total crystal mass will contain a wide size distribution of crystals ranging from seed crystals being formed, to those that are in the course of growth to product size. For many commercial purposes, the total crystal product would be unacceptable because of too many fines, oversize particles if the formation of larger crystals is permitted to go beyond specification size, and because of too wide a screen analysis.

Proper crystal size and shape are important specifications in many crystalline products, and it is especially necessary that the crystals be of uniform size for such purposes as appearance, resistance to cak-ing, ease of washing and uniform behavior in use.

Ideally, in any crystallization process the nuclei formation should be under control. In a continuous process the number of nuclei formed or presented per unit time should be continuous and uniform and should correspond generally with the number of product crystals withdrawn from the crystallizer during that time. In a continuous crystallizer, either most of the nuclei should form Within a narrow zone so that all nuclei can receive the same growth rate, or there must be a classifying action in the crystallizer which will retain the small crystals until they have grown to proper size.

From what has been stated, it will be understood that nucleation can be acomplished either by adding to the crystallizer the desired number of nuclei in the form of small crystals, or by precipitating them in situ. The present invention relates to that type of crystallizing technique involving the continuous making of carefully predetermined crystals which are employed as nuclei.

From what has been said in the foregoing, it will be apparent that in any crystal reduction for the formation of nuclei, the reduced crystals should have a high degree of uniformity and size, and that in any continuous system being fed with crop crystals from the crystallizer and operating to return the reduced crystals to the crystallizing zone therein, the process employed, for maximum elfectiveness and efficiency, should have the capacity to accomplish the crystal size reduction without necessitating the segregation or separation of particles which are not of the desired nuclei size. Also it is desirable that the nuclei particles in their reduced state retain the crystalline shapes characteristic of the material of which they are formed.

The invention has for its primary object to provide an improved process whereby a portion of the crop crystals may be continuously subjected to crushing in a manner productive of exceptional size uniformity, and desirable physical characteristics otherwise, in a single crushing operation the output of which is continuously returned to the crystallizer.

In accordance with the invention, a liquid slurry of the crop crystals flows through a passage formed between the inner surface of an outer tubular roll and the outer sur- 3 ,164,3Z8 Patented Jan. 5, 1965 face of an inner roll with an axial off-set of the outer roll so that the inner passage progressively merges to a location of narrow minimum clearance between the rolls. Importantly, the latter are rotated at substantially the same surface speeds and in the same direction as the slurry flow between them, so that the crop crystals become progressively crushed by essentially non-shearing compression to a dimension predetermined by the minimized roll clearance. Normally, the rolls may have cylindrical crushing surfaces, although the invention contemplates the possibility that such surface may be formed with circumferential or longitudinal corrugations.

According to my preferred practice, I employ an inner roll surface Whose diameter is greater than one-half the diameter of the outer roll surface, thus providing for gradual convergence of the surfaces and correspondingly gradual application of the crystal crushing forces. Such forces thus are compressive generally radially to the surfaces, to the exclusion of shearing forces that might undesirably rupture the crystals along cleavages which would tend to impair the characteristic crystalline shapes desired to be retained in the nuclei.

As will appear, the invention affords a continuous nuclei-formation process operable continuously in conjunction with any appropriate type of crystallizer, either externally thereof or directly within the crystallizer itself.

All the features and objects of the invention will be more fully understood from the following detailed description of an illustrative embodiment shown by the accompanying drawing in which:

FIG. 1 is a view illustrating diagrammatically the crushing roll assembly together with an associated crystallizer;

FIG. 2 is a view illustrating the crushing equipment with the outer roll appearing in longitudinal section, and FIG. 3 is a cross section on line 3-3 of FIG. 2.

Referring first to FIG. 1, it is contemplated that the crusher generally indicated at 10 shall operate in conjunction with any of various forms of commercial crystallizers, diagrammatically indicated at 11, either with the crusher rolls directly inside the crystallizer, or exteriorly thereof with appropriate crop feed and crushed interconnections. Typically, and for simplicity in illustration, a portion of the mother liquor slurry of crop crystals being removed from the crystallizer through line 12 is taken through line 13 as feed at the later described convergent crushing passage between roll surfaces in the crushing apparatus Ill, a slurry of the crushed crystals being fed through line 14 to supply said crystals for growth in the crystallizer 11.

The invention is applicable to the treatment of various crystalline materials, most generally in the category of crystallized organic and inorganic compounds, of which mono-di and tri-sodium phosphate, ammonium sulphate, sodium carbonate, sodium borate, potassium chloride, aspirin, sodium glutamate, are illustrative. As will be understood, the function of the crystallizer 11 is to promote growth of very small seed or nuclei crystals to so-called crop crystals of desired commercial size.

In reference to FIG. 2, the crushing equipment 10 is shown to comprise an outer cylindrical drum or roll 15 suitably mounted for rotation between stationary head plates 16 and 17, as by accommodating the ends of the drum within appropriate annular seals 1% for retaining the slurry during its flow through the equipment. As illustrative, the drum may be rotated by providing it with a ring gear 19 driven by motor 20 through shaft 21 and gear 22.

The drum 15 contains a smaller diameter, relatively axially offset cylindrical drum or roll 23 having a stub shaft 24 journaled in bearing 25, and an opposite end shaft 26 rotatable in bearing 27 and carrying gear 28 .35 which is driven by the motor through gears 29, 2&1, and pinion 31. 7

As previously observed, the diameter of the outer or crushing surface 23a of the inner roll 23 ordinarily will be substantially in excess of one-half the diameter of the inside crushing surface 15a of the outer roll 15. As illustrative, the diameter of surface 2311 may be in the neighborhood of four inches and the diameter of surface 15a about five inches.

By reason of the axially ofiset relation of the crushing surfaces, they form between them a gradually converging passage 32 into which the crop crystal slurry is fed from line 13 through inlet 33. Passage 32 converges to a location at 34 of minimum clearance between the roll surfaces and which is closely definitive of the size to which the crop crystals are reduced for entrainment in the slurry flowing through the enlarging passage 35 to outlet 36 connecting with line 14. The inlet 33 is shown to be positioned at the feed side of bafile 38 extending throughout the inter-roll space, and outlet 35 to be positioned elo-W a similar bathe 37.

It is contemplated that the gear ratios in the motor drive to the respective rolls shall be so determined that the crushing surfaces 15a and 23a Will rotate in the same direction at within about ten percdnt speed correspondence, and preferably at about the same peripheral speed. Where surfaces 23a and 3542 have diameters respectively of about four inches and five inches, their speed may be in the neighborhood of around 600 feet per minute.

It Will be observed that the passage 32 is of quite gradual coinvergence, as a consequence of which the enlarged feed crop crystals 36 become subjected to gradually progressive crushing forces between the surfaces with minimized tendency for shearing of the crystals, by reason of correspondence in the crushing surface speeds. The crop crystals therefore become gradually reduced primarily by force application generally radially of the rolls, so as to maintain desirable crystalline configurations.

Merely for purposes of clarity and illustration the clearance at 34 appears appropriately greater than would be the spacing in actual practice. For the production of seed crystals the most practicably suitable as nuclei in the crystallizer, the minimum clearance at 34 Will be in the range of about 0.003 to 0.030 inch. In this manner the feed crystals become only progressively reduced by reason of the gradual convergence of passage 32 and rotation of the crushing surfaces in the flow direction, in a manner avoiding abrupt crushing of the large crystals so as ultimately reduced, the crop crystals tend toward uniformity in size and characteristic crystalline shape. Thus, to cite a specific example, feed or crop crystals having a screen analysis of 30% of +35 mesh were reduced to seed crystals having a screen analysis of of 35 to 100 mesh with 1% of +35 mesh, with the respectively four inch and five inch rolls having three inch Width faces with 0.015 inch clearance at 34, were operated at a crystal throughput of about pounds per hour.

I claim:

1. The ncthod of producing seed crystals from relatively larger crop crystals, that includes,

(A) flowing a liquid slurry of the crop crystals through a passage formed between the inner surface of an outer tubular roil and the outer surface of an inner roll Within and axially offset from the outer roll so that said passage progressively converges to a location of narrow minimum clearance between the rolls,

(B) rotating said surfaces at substantially the same speed and in the direction of the slurry iiow between them so that the crop crystals become progressively crushed by direct non-shearing compression between the suriaces to a dimension predetermined by said clearance,

(C) conducting the crushed crystal slurry to a crystallizing Zone and therein forming enlarged crystals using the crushed crystals as seed, and

(D) passing a slurry of the enlarged crystals as crOp crystal feed to said passage.

2. The method of claim 1, in which said clearance is in the order of about 0.003 to 0.030 in.

3. The method of claim 2, in which the radius of said inner surface is greater than one-half the radius of said outer surface.

4. The method of claim 3, in which said crystals are of the group consisting of organic and inorganic crystals slurried in their mother liquor, and in which said clcarance is in the order of about 0.003 to 0.030 in.

References Cited by the Examiner UNITED STATES PATENTS 1,503,157 7/24 I-Ialliday 241228 X 2,220,153 11/40 Hoge 241-22S 2,748,667 6/56 Grieb et al 24l228 X ANDREW R. IUHASZ, Primary Examiner. 

1. THE METHOD OF PRODUCING SEED CRYSTALS FROM RELATIVELY LARGER CROP CRYSTALS, THAT INCLUDES, (A) FLOWING A LIQUID SLURRY THE THE CROP CRYSTALS THROUGH A PASSAGE FORMED BETWEEN THE INNER SURFACE OF AN OUTER TUBULAR ROLL AND THE OUTER SURFACE OF AN INNER ROLL WITHIN AND AXIALLY OFFSET FROM THE OUTER ROLL SO THAT SAID PASSAGE PROGRESSIVELY CONVERGES TO A LOCATION OF NARROW MINIMUM CLEARANCE BETWEEN THE ROLLS, (B) ROTATING SAID SURFACES AT SUBSTANTIALLY THE SAME SPEED AND IN THE DIRECTION OF THE SLURRY FLOW BETWEEN THEM SO THAT THE CROP CRYSTALS BECOME PROGRESSIVELY CRUSHED BY DIRECT NON-SHEARING COMPRESSION BETWEEN THE SURFACES TO A DIMENSION PREDETERMINED BY SAID CLEARANCE, (C) CONDUCTING THE CRUSHED CRYSTAL SLURRY TO A CRYSTALLIZING ZONE AND THEREIN FORMING ENLARGED CRYSTALS USING THE CRUSHED CRYSTALS AS SEED, AND (D) PASSING A SLURRY OF THE ENLARGED CRYSTALS AS CROP CRYSTAL FEED TO SAID PASSAGE. 